1. 1Health Protection Agency, South East Regional Epidemiology Unit, London SW1W 9SZ, UK
2. 2European Programme for Intervention Epidemiology Training, European Centre for Disease Prevention and Control, Stockholm SE-17183, Sweden
3. 3Health Protection Agency, Surrey and Sussex Health Protection Unit, Horsham RH12 1XA, UK
4. 4Health Protection Agency, Hampshire and Isle of Wight Health Protection Unit, Whiteley PO15 7FN, UK